Precharge circuit using non-regulating output of an amplifier

ABSTRACT

A reference signal generator includes a voltage reference, an amplifier coupled to the voltage reference, and a precharge circuit coupled to the amplifier. The voltage reference is configured to generate a constant voltage. The amplifier is configured to receive the constant voltage from the voltage reference and generate a regulating primary output signal and a non-regulating secondary output signal. The precharge circuit is configured to charge a noise reduction capacitor with the non-regulating secondary output signal.

BACKGROUND

Linear voltage regulators are used in many electronic devices tomaintain a constant voltage to drive a load. Because the load may vary,the resistance of the regulator varies based on the load so that aconstant voltage is produced. One type of linear voltage regulator is alow-dropout (LDO) regulator. LDO regulators are configured to regulatevoltage even if the supply voltage is close to the regulated outputvoltage. LDO regulators include a differential amplifier that drives apower transistor. One input of the differential amplifier is a referencevoltage generated by a voltage reference (e.g., a bandgap reference, aZener diode, etc.). The second input of the differential amplifier is afraction of the output voltage of the LDO regulator. Thus, the drivevoltage to the power transistor changes to regulate the output voltagebased on the value of the output voltage itself. In other words, if theoutput voltage rises relative to the reference voltage, the drivevoltage powering the power transistor also changes to maintain an outputvoltage that remains constant.

SUMMARY

In accordance with at least one embodiment of the disclosure, areference signal generator includes a voltage reference, an amplifiercoupled to the voltage reference, and a precharge circuit coupled to theamplifier. The voltage reference is configured to generate a constantvoltage. The amplifier is configured to receive the constant voltagefrom the voltage reference and generate a regulating primary outputsignal and a non-regulating secondary output signal. The prechargecircuit is configured to charge a noise reduction capacitor with thenon-regulating secondary output signal.

Another illustrative embodiment is a precharge circuit that includes acapacitor charging circuit, a reference signal generation circuit, and acontrol circuit. The capacitor charging circuit is configured to chargea noise reduction capacitor with a non-regulating secondary outputsignal received from an amplifier. The reference signal generationcircuit is configured to generate a reference signal based on aregulating primary output signal received from the amplifier. Thecontrol circuit is configured to enable and disable the non-regulatingsecondary output signal.

Yet another illustrative embodiment is a method for generating areference signal. The method includes generating, by an amplifier, aregulating primary output signal. The method also includes generating,by the amplifier, a non-regulating secondary output signal. The methodalso includes charging a noise reduction capacitor with thenon-regulating secondary output signal. The method also includesgenerating a reference signal with the regulating primary output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows an illustrative block diagram of a reference signalgenerator in accordance with various examples;

FIG. 2 shows an illustrative block diagram of a precharge circuit in areference signal generator in accordance with various examples;

FIG. 3 shows an illustrative circuit diagram for an amplifier andprecharge circuit in a reference signal generator in accordance withvarious examples; and

FIG. 4 shows an illustrative flow diagram of a method for generating areference signal in accordance with various examples.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, companies may refer to a component by different names. Thisdocument does not intend to distinguish between components that differin name but not function. In the following discussion and in the claims,the terms “including” and “comprising” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to . . . .” Also, the term “couple” or “couples” is intended tomean either an indirect or direct connection. Thus, if a first devicecouples to a second device, that connection may be through a directconnection, or through an indirect connection via other devices andconnections. The recitation “based on” is intended to mean “based atleast in part on.” Therefore, if X is based on Y, X may be based on Yand any number of other factors.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of thedisclosure. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure, including the claims. Inaddition, one skilled in the art will understand that the followingdescription has broad application, and the discussion of any embodimentis meant only to be exemplary of that embodiment, and not intended tointimate that the scope of the disclosure, including the claims, islimited to that embodiment.

Linear voltage regulators are used in many electronic devices tomaintain a constant voltage to drive a load. Because the load may vary,the resistance of the regulator varies based on the load, so that aconstant voltage is produced. As discussed above, one type of linearvoltage regulator is a LDO regulator which includes a differentialamplifier that compares a reference voltage to the output voltage todrive a power transistor, thus, regulating the output voltage.

Although linear regulators do not generate switching noise, they doinclude device noise. Device noise is an unavoidable consequence of thequantum nature of charge carriers. Device noise varies with frequency,so it is usually analyzed in terms of noise spectral density. In manydatasheets, the root-mean-square (RMS) noise voltage over a specificband of frequencies (e.g., 10 Hz-100 kHz) is specified. To generate thereference voltage, linear regulators use a voltage reference (e.g., abandpass reference, a Zener diode, etc.). However, these voltagereferences suffer from flicker noise (e.g., 1/f noise). The noisespectral density of flicker noise varies inversely with frequency andtypically dominates the noise in the RMS noise voltage. Flicker noisecan be suppressed with a resistor-capacitor (RC) filter; however, forbest results, the corner frequency of the RC filter should be at leastan order of magnitude beneath the lower edge of the bandwidth ofinterest. For example, if the RMS noise voltage is between 10 Hz and 100kHz, the RC filter corner frequency should be no greater than 1 Hz.Thus, the capacitor in the RC filter needs to be relatively large, andin many cases, cannot be onboard with the voltage reference and the RCfilter resistor. Due to the size of the capacitor, the settling time ofsuch an RC filter is objectionably large (e.g., several seconds) unlessa precharge circuit is utilized to precharge the capacitor (known as anoise reduction capacitor).

One conventional precharge circuit bypasses a filter resistor with aswitch. When the switch is closed, the noise reduction capacitor isprecharged with a relatively large current through the switch. Once theprecharge period is complete, the switch is opened, and the outputvoltage of the voltage reference is filtered by the filter resistor andthe noise reduction capacitor. However, this type of conventionalprecharge circuit causes voltage droops in the voltage reference duringthe precharge period. In other words, precharging the noise reductioncapacitor causes the voltage reference to collapse. Because the voltagereference may provide a reference voltage to other circuitry, thevoltage droop caused by precharging the noise reduction capacitor isunacceptable. To cure this deficiency, some conventional prechargecircuits include a buffer amplifier between the voltage reference andthe noise reduction filter (e.g., the filter resistor and noisereduction capacitor). While this does solve some of the problemsdiscussed above (e.g., prevents the collapse in the voltage reference),other problems arise. For example, while many linear regulators includethe external noise reduction capacitor, the regulator is also expectedto work without this capacitor. Therefore, in this configuration, thebuffer amplifier must be internally compensated, which can be difficultto design. Furthermore, in some systems, the buffer amplifier itself canbe used to provide a voltage to other circuitry, thus, the voltage droopin the output signal generated by the amplifier caused by prechargingthe noise reduction capacitor in such configurations, is unacceptable.Therefore, there is a need for a reference signal generator that doesnot cause the droop in the voltage generated by the voltage referenceitself and/or the voltage generated by the buffer amplifier and stillprecharges the noise reduction capacitor to provide filtering of thereference voltage generated by the circuit.

In accordance with various examples, a reference signal generator isprovided that precharges a noise reduction capacitor while preventingvoltage droop in the outputs of the voltage reference and/or the bufferamplifier. The buffer amplifier is configured to generate a regulatingprimary output signal for use in generating the reference voltage and asthe negative feedback for the amplifier. The buffer amplifier is alsoconfigured to generate a non-regulating secondary output signal at avoltage which tracks the voltage of the regulating primary outputsignal. In other words, the amplifier generates a parasitic paralleloutput voltage to the regulating primary output signal. Thenon-regulating secondary output signal acts to precharge the noisereduction capacitor without affecting the regulating primary outputsignal. Once the noise reduction capacitor is charged, thenon-regulating secondary output signal can be disabled. The noisereduction capacitor along with a filter resistor act as an RC filter toreduce noise in the regulating primary output signal and generate thereference voltage with a reduced amount of noise. Because the regulatingprimary output signal is not used to charge the noise reductioncapacitor, there is no voltage droop in that output. Therefore theregulating primary output can also be utilized by any circuitry at anytime, including while the noise reduction capacitor is being precharged.Once the precharge is complete, the filtered regulating primary outputsignal is then output as the reference voltage.

FIG. 1 shows an illustrative block diagram of a reference signalgenerator 100 in accordance with various examples. The reference signalgenerator 100 includes, in an embodiment, a voltage reference 102, abuffer amplifier 104, and a precharge circuit 106. The voltage reference102 is coupled to the buffer amplifier 104 which is coupled to theprecharge circuit 106. The voltage reference 102 is configured togenerate a constant voltage signal 122. The voltage reference 102 is, inan embodiment, a bandpass reference; however, in alternativeembodiments, the voltage reference 102 can be any type of voltagereference (e.g., a Zener diode). The amplifier 104 is configured toreceive the constant voltage signal 122 from the voltage reference 102and generate a regulating primary output signal 124 and a non-regulatingsecondary output signal 126. The regulating primary output signal 124acts, in an embodiment, as the negative feedback signal for theamplifier 104. The non-regulating secondary output signal 126 can act asa parasitic parallel output voltage that tracks the voltage of theregulating primary output signal 124.

The precharge circuit 106 is configured to receive the regulatingprimary output signal 124 and the non-regulating secondary output signal126. The precharge circuit 106 is also configured to charge a noisereduction capacitor, which, in some embodiments, is a part of theprecharge circuit 106, with the non-regulating secondary output signal126. The precharge circuit 106 can enable and disable the non-regulatingsecondary output signal 126. For example, the precharge circuit 106 canenable the non-regulating secondary output signal 126 to precharge thenoise reduction capacitor, and, once the noise reduction capacitor ischarged, disable the non-regulating secondary output signal 126. Oncethe noise reduction capacitor is charged, the precharge circuit 106generates reference signal 128 by, in some embodiments, filtering theregulating primary output signal 124 with the noise reduction capacitorand a filter resistor, together acting as an RC filter. The referencesignal 128 then can act as a voltage reference for one or more circuits(e.g., the voltage reference of a differential amplifier as part of aLDO regulator).

FIG. 2 shows an illustrative block diagram of precharge circuit 106 inreference signal generator 100 in accordance with various examples. Theprecharge circuit 106, in an embodiment, includes capacitor chargingcircuit 202, reference signal generation circuit 204, and controlcircuit 206. The capacitor charging circuit 106 is configured to receivethe non-regulating secondary output 126 and charge the noise reductioncapacitor with the non-regulating secondary output 126. The referencesignal generation circuit is configured to generate the reference signal128 based on the received regulating primary output 124. The controlcircuit 206 is configured to enable and/or disable the non-regulatingsecondary output signal 126. In some embodiments, the control circuit206 is configured to disable the non-regulating secondary output signal126 in response to a determination that the noise reduction capacitor ischarged by the non-regulating secondary output 126.

FIG. 3 shows an illustrative circuit diagram for amplifier 104 andprecharge circuit 106 in reference signal generator 100 in accordancewith various examples. The example amplifier 104 shown in FIG. 3,includes transistors 302-310, a resistor divider that comprisesresistors 316-318, and an internal compensation capacitor 314. Although,shown in FIG. 3 as a low-current transistor amplifier stabilized by theinternal compensation capacitor 314, in other embodiments, the amplifier104 can be designed with alternative components and/or connections(e.g., a cascode amplifier, etc.). As discussed above, the prechargecircuit 106 includes, in an embodiment, capacitor charging circuit 202,reference signal generation circuit 204, and control circuit 206. Thecapacitor charging circuit 202 can include the transistors thetransistors 330-332, the load resistor 342, and the noise reductioncapacitor 346, which may be connected to the remaining components of thecapacitor charging circuit 202 through pin 344. The reference signalgeneration circuit 204 can include the filter resistor 340 and the noisereduction capacitor 346 connected together in series through, in someembodiments, pin 344. The control circuit 206 can include transistors334-336, and inverter 338. While the transistors 302-310 and 330-336 aredepicted as metal-oxide-semiconductor field-effect transistors (MOSFETs)in FIG. 3, they can be any type of transistor, including bipolarjunction transistors (BJTs).

The amplifier 104 depicted in FIG. 3 is highly stable because the filterresistor 340 from the reference signal generation circuit 204 decouplesthe noise reduction capacitor 346 from the amplifier 104, thus, allowingthe internal compensation capacitor 314 to provide dominant polecompensation regardless of whether the noise reduction capacitor 346 ispresent or not. The output at the source of transistor 310, which insome embodiments is an n-channel MOSFET, is the regulating primaryoutput signal 126. The negative feedback loop of the amplifier 106 isclosed through the resistor divider consisting of resistors 316-318which attenuates the regulating primary output signal 126, and thus, thereference signal 128 to match the constant voltage signal 122 generatedby the voltage reference 102. Additionally, this arrangement enables theamplifier 104 to generate a reference signal 128 that has a largervoltage than the constant voltage signal 122 which can be desirable inlinear regulators because the reference signal 122 can be scaled toequal the desired output voltage of the regulator to eliminate the needfor a noisy resistor divider within the regulator itself. The noisegenerated by the resistor divider consisting of resistors 316-318 issignificantly reduced, and in some embodiments, effectively eliminated,by the RC filter in the reference signal generation circuit 204 which,when the noise reduction capacitor 346 is charged, filters theregulating primary output signal 126 to generate the reference signal128. Therefore, scaling the reference signal 128 in this manner reducesthe overall device noise in the system (e.g., the regulator).

The resistance value of the resistance divider comprising resistors316-318 is set, in an embodiment, based on the amount of bias currentthat is desired (or acceptable) to be generated and/or wasted. Forexample, if a very small amount of bias current can be wasted, thesummation of the resistance value of resistors 316 and 318 may be verylarge (e.g., multiple MΩs). While the resistor divider shown in FIG. 3includes two resistors, in some embodiments resistor 316 is not presentand/or the resistance value of resistor 316 is set to 0.

To precharge the noise reduction capacitor 346, the signal 352 causesthe transistor 330 (acting as a switch) to close (turn on). As shown, inFIG. 3, the transistor 330 is a p-channel MOSFET; therefore, the signal352 is driven LOW closing the transistor 330. When the transistor 330 isclosed, transistor 332, which in some embodiments is an n-channelMOSFET, is connected to the amplifier 104, and more specifically thegate of transistor 310. Transistor 332 acts as an open-loop output thatdirectly drives (precharges) the noise compensation capacitor 346. Inother words, because the gate of transistor 332 is connected directly tothe gate of transistor 310 when the transistor 330 is closed, aparasitic output, the non-regulating secondary output signal 126, isformed parallel to the regulating primary output signal 124. Therefore,closing the transistor 330 enables the non-regulating secondary outputsignal 126. Because the feedback loop of the amplifier 104 is closedfrom transistor 310, through the resistor divider consisting of resistor316-318, and not from transistor 332, the closing of transistor 330 doesnot upset the stability of the amplifier 104.

While the noise reduction capacitor 346 is being precharged, the loadresistor 342 provides a load on transistor 332. If transistor 332 is thesame size as transistor 310, then resistor 342 is configured to have thesame resistance as the sum of resistors 316 and 318. If the width oftransistor 332 is different (e.g., wider) than the width of transistor310, the resistance of the load resistor 342 is configured, in anembodiment, according to: R₃₄₂=W₃₁₀/W₃₃₂ (R₃₁₆+R₃₁₈) where R₃₄₂ is theresistance of load resistor 342, R₃₁₆ is the resistance of resistor 316,R₃₁₈ is the resistance of resistor 318, W₃₁₀ is the width of transistor310, and W₃₃₂ is the width of transistor 332. In this way, the finalgate-to-source voltage drop of transistor 332 (after the noise reductioncapacitor 346 is charged) will equal the gate-to-source voltage oftransistor 310. In other words, load resistor 342 provides the same ornearly the same load on transistor 332 as the load the resistor dividermade up of resistors 316-318 provides to transistor 310. Therefore,transistor 332 has the approximately the same current draw through it asthe current draw through transistor 310. In this way, the voltage of thenon-regulating secondary output signal 126 tracks the voltage of theregulating primary output signal 124. Thus, the voltage to which thenoise reduction capacitor 346 settles during precharge is the same asthe voltage at which it will operate when precharge terminates.

Once the noise reduction capacitor 346 is charged, the control circuit206, through the signal 352, is configured to open (turn off) thetransistor 330. In other words, a HIGH signal 352 drives the gate oftransistor 330 opening the transistor 330. This causes the amplifier 104to stop generating the non-regulating secondary output signal 126 (i.e.,disables the non-regulating secondary output signal 126). The RC filterformed by the filter resistor 340 and the noise reduction capacitor 346,then acts to filter the regulating primary output signal 124 to generatethe reference signal 128 as discussed above. The filter resistor 340has, in an embodiment, a resistance value based on the amount of pinleakage from pin 344, and thus, in some embodiments a maximum resistancevalue of 100 kn. For example, if the leakage current of the pin 344 is0.1 μA and the resistance value of filter resistor 340 is 1 MΩ, then thevoltage drop is 100 mV which is unacceptable. However, if the resistancevalue of the filter resistor 340 is 100 kΩ, then the voltage drop is 10mV which may be tolerable in certain applications. If the pin leakagecurrent of the pin 344 is very low (e.g., 0.02 μA), then higherresistance values are available for the filter resistor 340 (e.g., 1MΩ).

FIG. 4 shows an illustrative flow diagram of a method 400 for generatinga reference signal in accordance with various examples. Though depictedsequentially as a matter of convenience, at least some of the actionsshown can be performed in a different order and/or performed inparallel. Additionally, some embodiments may perform only some of theactions shown. In some embodiments, at least some of the operations ofthe method 400, as well as other operations described herein, areperformed by the voltage reference 102, the amplifier 104, and/or theprecharge circuit (including the capacitor charging circuit 202, thereference signal generation circuit 204, and/or the control circuit 206)and implemented in logic.

The method 400 begins in block 402 with generating a regulating primaryoutput signal. For example, the buffer amplifier 104 is configured toreceive constant voltage signal 122 and generate regulating primaryoutput signal 124, which, in some embodiments, is an amplified versionof the constant voltage signal 122. The regulating primary output signal124 is also used as the negative feedback input to the amplifier 124. Inblock 404, the method 400 continues with generating a non-regulatingsecondary output signal. For example, in addition to generating theregulating primary output signal 124, the amplifier 104 is alsoconfigured to generate non-regulating secondary output signal 126 with avoltage which tracks the voltage of the regulating primary output signal124. In other words, the amplifier 104 generates a parallel parasiticvoltage that tracks the voltage of the regulating primary output signal124.

The method 400 continues in block 406 with charging a noise reductioncapacitor with the non-regulating secondary output signal. For examplecapacitor charging circuit 202 is configured to charge the noisereduction capacitor 346. In block 408, the method 400 continues withdetermining whether the noise reduction capacitor is fully charged. If,in block 408, a determination is made that the noise reduction capacitoris not fully charged, the method 400 continues in block 406 withcontinuing to charge the noise reduction capacitor with thenon-regulating secondary output signal.

However, if, in block 408, a determination is made that the noisereduction capacitor is fully charged, the method 400 continues in block410 with disabling the non-regulating secondary output signal. Forexample, the control circuit 206 is configured to disable thenon-regulating secondary output signal by, in some embodiments, drivingthe gate of transistor 330 to turn it off.

In block 412, the method 400 continues with filtering the regulatingprimary output signal to generate a reference signal. For example, thenoise reduction capacitor 346 in conjunction with a series filterresistor 340 filter the regulating primary output signal 124 to generatethe reference signal 128.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present disclosure. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. A reference signal generator, comprising: avoltage reference configured to generate a constant voltage; anamplifier coupled to the voltage reference, the amplifier configured toreceive the constant voltage from the voltage reference and generate aregulating primary output signal and a non-regulating secondary outputsignal; and a precharge circuit coupled to the amplifier, the prechargecircuit configured to charge a noise reduction capacitor with thenon-regulating secondary output signal.
 2. The reference signalgenerator of claim 1, wherein the precharge circuit is furtherconfigured to enable and disable the non-regulating secondary outputsignal.
 3. The reference signal generator of claim 2, wherein theprecharge circuit is further configured to enable the non-regulatingsecondary output signal to charge the noise reduction capacitor, and inresponse to the noise reduction capacitor being charged, disable thenon-regulating secondary output signal.
 4. The reference signalgenerator of claim 3, wherein the precharge circuit includes a switchand a control circuit, the control circuit configured to: close theswitch to enable the non-regulating secondary output signal to chargethe noise reduction capacitor; and open the switch to disable thenon-regulating secondary output signal.
 5. The reference signalgenerator of claim 4, wherein the switch is a p-channelmetal-oxide-semiconductor field-effect transistor (MOSFET).
 6. Thereference signal generator of claim 1, wherein the precharge circuit isfurther configured to receive the regulating primary output signal andgenerate a reference signal.
 7. The reference signal generator of claim6, wherein the precharge circuit includes a resistor in series with thenoise reduction capacitor, the resistor and noise reduction capacitorconfigured to filter the regulating primary output signal to generatethe reference signal.
 8. The reference signal generator of claim 1,wherein a voltage of the non-regulating secondary output signal tracks avoltage of the regulating primary output signal.
 9. A precharge circuit,comprising: a capacitor charging circuit configured to charge a noisereduction capacitor with a non-regulating secondary output signalreceived from an amplifier; a reference signal generation circuitconfigured to generate a reference signal based on a regulating primaryoutput signal received from the amplifier; and control circuitconfigured to enable and disable the non-regulating secondary outputsignal.
 10. The precharge circuit of claim 9, further comprising a firstresistor in series with the noise reduction capacitor, the resistor andnoise reduction capacitor configured to filter the regulating primaryoutput signal to generate the reference signal.
 11. The prechargecircuit of claim 9, wherein the capacitor charging circuit includes afirst switch configured to, when closed, enable the non-regulatingsecondary output signal to charge the noise reduction capacitor and,when open, disable the non-regulating secondary output signal.
 12. Theprecharge circuit of claim 11, further comprising a first resistor inseries with the noise reduction capacitor, the resistor and noisereduction capacitor configured to, in response to the first switch beingopen, filter the regulating primary output signal to generate thereference signal.
 13. The precharge circuit of claim 11, wherein thecapacitor charging circuit further includes a second switch connected tothe first switch, the second switch configured to, in response to thefirst switch being closed, drive the noise reduction capacitor.
 14. Theprecharge circuit of claim 13, wherein the capacitor charging circuitfurther includes a first load resistor connected to the second switch,the first load resistor configured to provide a load to thenon-regulating secondary output signal so that the non-regulatingsecondary output signal tracks the regulating primary output signal. 15.The precharge circuit of claim 14, wherein the first switch is ap-channel metal-oxide-semiconductor field-effect transistor (MOSFET) andthe second switch is an n-channel MOSFET.
 16. The precharge circuit ofclaim 15, wherein: the first switch includes a first drain, a firstsource, and a first gate; the second switch includes a second drain, asecond source, and a second gate; the first source is connected to agate of an amplifier transistor included in the amplifier; the firstdrain is connected to the second gate; and the second source isconnected to the noise reduction capacitor.
 17. The precharge circuit ofclaim 16, wherein the second source is further connected to the firstload resistor.
 18. The precharge circuit of claim 16, wherein thereference signal generation circuit includes a second load resistorconnected to a source of the amplifier transistor and the noisereduction capacitor.
 19. A method for generating a reference signal,comprising: generating, by an amplifier, a regulating primary outputsignal; generating, by the amplifier, a non-regulating secondary outputsignal; charging a noise reduction capacitor with the non-regulatingsecondary output signal; and generating a reference signal with theregulating primary output signal.
 20. The method of claim 19, furthercomprising: in response to a determination that the noise reductioncapacitor is charged, disabling the non-regulating secondary outputsignal; and filtering the regulating primary output signal.